Energy saving is a big issue nowadays due to the requirement of decreasing use of fossil fuels and required reduction in emission of Green House Gases (GHG) to slow down climate change. Energy saving includes efficient energy use and using renewable energy. Renewable energy sources are those that produce energy without depleting resources. Renewable energy includes solar, wind, water, air, earth and biomass power, and energy from waste. Using renewable energy is the most significant means to reduce GHG and save fossil fuel resources. In the application of renewable energy, a very important aspect is the storage of energy, especially heat storage when applying solar, waste heat, air (air source heat pumps), etc.
The materials used for thermal storage have been developed for decades. Heat storage materials absorb heat through standard heat transfer mechanisms such as radiation, conduction, and convection. As the temperature in environment drops, they subsequently release the stored heat in the same fashion. Active space heating systems commonly use tanks of water or bins of rock as a thermal storage material. Under normal conditions, water has the biggest specific heat capacity of 1.161 wh/(Kg·K) among other materials, so it is the typical heat storage medium in solar water heating systems and other heat storage systems. These heat storage materials deal with “sensible” heat. This means as they absorb heat, their temperature increases and they become hotter.
Another type of thermal storage materials is Phase Change Materials (PCMs) which use the “latent” heat to store thermal energy. Solid-liquid PCMs are conventional and practical PCMs. The thermal energy transfer occurs when a material changes from a solid to a liquid or from a liquid to a solid. This is called a change in state, or “phase”. In fact, PCMs use chemical bonds to store and release heat. Initially, these solid-liquid PCMs perform like conventional storage materials; their temperature rises as they absorb heat. Unlike conventional “sensible” heat storage materials, when PCMs reach the temperature at which they change phase (their melting points) they absorb large amounts of heat without increasing temperature. When the temperature in environment around the PCM drops, the PCM solidifies, releasing its stored latent heat. PCMs absorb and release heat while maintaining a nearly constant temperature. Theoretically they can store several times more heat per unit volume than “sensible” heat storage materials such as water and rock.
PCMs can be classified as inorganic PCMs and organic PCMs. Glauber's salt (sodium sulfate decahydrate), calcium chloride hexahydrate, and sodium acetate are the most common inorganic PCMs that are considered to have a possible application. Paraffin wax is the most common organic PCM.
The heat storage materials absorb heat and release heat, so that they can control temperature. Therefore these materials can be referred to as Temperature Control Materials (TCM) based on their functions of temperature control.
Up to now, few PCMs are practically being applied in temperature control and heat storage, especially used in temperature control within the human comfort range of 16 to 28° C. The reasons are mainly:
1. Because the chemicals in inorganic PCMs separate and stratify when in their liquid state, the PCMs have not always re-solidified properly. When temperatures dropped, they did not completely solidify, thus reducing their capacity to store latent heat.
2. A eutectic composition is required to constitute to reach temperature control range of human comfort temperature of 16-28° C.
3. Organic PCMs overcome the disadvantages of inorganic PCMs, but they perform lower latent heats, have high costs and flammable.
Among inorganic PCMs, pure sodium sulfate decahydrate (Na2SO4.10H2O) possesses a potential latent heat of about 100 Kwh/m3, melting point of 32.4° C., and density of 1.46 g/cm3. In addition, it is inexpensive, available in large quantities and safe to use. Therefore, sodium sulfate decahydrate as a main ingredient would offer the best prospect for possible application in temperature control for human comfort level of 16 to 28° C. In fact, however, sodium sulfate decahydrate as a TCM used in human comfort level has the following problems to be overcome: one problem is its tendency to super-cooling releasing large quantities of heat at undetermined times; second problem is the solid separation of anhydrous sodium sulfate (density of 2.68 g/cm3), consequently reducing ability to store heat. Furthermore, a eutectic composition is required to constitute to reach the temperature of human comfort level.
Super-cooling can be solved by the addition of a nucleating agent such as borax, as disclosed, for example, in U.S. Pat. No. 2,677,664 to Telkes, U.S. Pat. No. 3,986,969 to Telkes, and U.S. Pat. No. 4,237,023 to Johnson et al. Thus, super-cooling temperature can be within the range of human comfort temperature level, therefore it is not an important problem.
Eutectic compositions having melting points at temperature of 16 to 28° C. are not difficult to constitute. It can be done by the addition of fusion (melting) temperature-depressing salts that are generally non-hydrated salts, are conventionally employed in amounts such that the resulting binary system is a eutectic mixture, as disclosed in U.S. Pat. No. 3,986,969 to Telkes, U.S. Pat. No. 4,619,778 to Chalk et al, and U.S. Pat. No. 5,453,213 to Kakiuchi et al. The common fusion temperature-depressing salts are sodium chloride, potassium chloride, ammonium chloride, ammonium sulfate, etc.
The most serious problem is the separation and stratification of solid anhydrous sodium sulfate to form a hard deposit layer when in liquid state such largely reducing formation of decahydrate, then dramatically reducing the heat storage capacity. This problem has been proposed to solve by adding thickening agents, or packaging the composition in thin or shallow containers. For example, U.S. Pat. No. 3,986,969 to Telkes discloses that attapulgus-type clay (magnesium aluminum silicate) is used as a homogenizing or thickening agent. Johnson et al, in U.S. Pat. No. 4,237,023, discloses fumed silicon dioxide is acted as a stabilizing agent, and the composition is packaged in a water vapor-impermeable container in which the thickness is limited to a thickness that allows recrystallization of the composition to occur primarily by diffusion. U.S. Pat. No. 4,273,667 to Kent, et al proposes a hydrogel comprising a water-swollen cross-linked polymer to be used as a thickening agent. U.S. Pat. No. 4,619,778 to Chalk et al also discloses a water-swollen cross-linked polymer hydrogel as a thickening agent. Kakiuchi, et al, in U.S. Pat. No. 5,453,213 claims to use a thickening agent selected from the group consisting of carboxymethyl cellulose, attapulgite clay and water-insoluble hydrogels for their composition. Voisinet et al, in U.S. Pat. No. 4,747,240 discloses that capsules consisting of their composition used as a building material are approximately spherical in shape and of a diameter of about 500 to about 3,000 microns. U.S. Pat. No. 4,287,076 to Babin et al describes that the composition is dispersed in an oil to which an emulsifying agent has been added to prevent from the solid separation and stratification.
Despite the improved ability achieved by a lot efforts in the prior art, these compositions still do not have sufficient retention of heat storage efficiencies to make them practical, such sufficient problems remain that significant application has not occurred.
The following patents and Reference are cited:
U.S. Pat. Nos.:2,677,664Telkes3,986,969Telkes4,237,023Johnson et al4,619,778Chalk et al5,453,213Kakiuchi et al4,273,667Kent et al4,747,240Voisinet et al4,287,076Babin et al